Propagation of electromagnetic waves through materials with varying properties such as dielectric permittivity, conductivity and permeability, was and remains in the focus of fundamental research due to its enormous value for wireless communication and sensing systems. Due to high efficiency of the radiation from relatively small transmitting facilities, the high-frequency waves are usually those mostly used in communication. However, the ranges the waves can reach are limited by skin depth which usually scales inversely as square root of frequency. On the other hand, moving towards low-frequency has certain benefits such as deeper penetration and lower scattering sensitivity to the objects which are smaller as compared to the wavelength of the signals. One of the main challenges is the size of the radiating source which ordinarily has to be commensurate with the wavelength of the radiated waves in order to achieve acceptable level of radiation resistance.
Directivity is a figure of merit for antenna and it measures the power density P(θ, ϕ) the antenna radiates in the direction of its strongest emission relative to the power density of the same antenna averaged over the entire solid angle Ω(θ, ϕ):Dir(θϕ)=max {P(θϕ)/(∫dΩP(θϕ))}
The commonly used setting of a loop antenna implies maximum gain at θ=0 assuming the plane of the radiating loop is normal to the z-axis, the latter will conventionally be considered as the radiator axis. One of the traditional methods to increase the directivity is to place the loop over a planar reflector. For this setting, the directivity is about 9 dB for spacings between the loop and the reflector in the range 0.005≤dλ≤0.2, where d is distance between the loop and the reflector and λ is the wavelength. In general, the directivity depends on the size, shape and the conductivity of the reflector and is a matter of optimization of those parameters as well, but the directivity stays around 9 dB at the maximally optimized range of the parameters, which is s/λ˜1 for a square shaped perfectly conducting reflector, with s standing for the length of its side.
Another method of controlling directivity is using a coaxial array in which all the loops are parallel and have their centers on a common axis. Here, the controlling parameter is ratio between the loop length and the wavelength, 2πr/λ, where r is the loop radius and λ is the wavelength. This method is efficient if the parameter 2πr/λ is close to unit. In this case, the induced currents in all loops have nearly same phase and hence there is no cancellation of the generated electromagnetic field. The mostly used configuration includes a single driven loop and several parasitic loops, in which case the feed arrangement needed to obtain the prescribed driving-point voltages can easily be obtained. If the size of the parasitic loop is slightly smaller than the wavelength (or the size of the driven loop), typically 2πrdirector/λ˜0.95, then directivity gains its maximum of about 7 dB on the side of the parasitic loop, the latter therefore considered as a director. If size of the parasitic loop is slightly smaller than the wavelength (or the size of the driven loop), typically 2πrdirector/λ˜1.05, then directivity gains its maximum of about 7 dB on the side opposite to the parasitic loop, the latter therefore considered as a reflector. Spacing between the driven loop and parasitic loops is another controlling parameter. Spacing of dλ˜0.2 is considered as optimum for achieving maximum directivity. The physics behind the array setting to maximize the radiation directivity is related to the differences between phases of the probing voltage at the location of the parasitic loop and the current induced by the voltage: if parasitic loop is smaller than this difference is negative, and vice-versa. The interference between fields from all elements of the array results in the distortion of the field pattern and asymmetry with respect to θ=0 and θ=π directions, i.e. enhancement of the directivity.
Situation changes dramatically when there is a wave-compressing medium into which the driven loop is immersed, and this is the configuration which the current invention is addressing. Due to the fact that conditions of the wave compression, i.e. when the insulated driven loop is immersed into the cavity characterized by certain dimensions, shape and material parameters, can in general be destroyed by presence of other objects around, requires special consideration in order to preserve wave-compressing and achieve high directivity gain at the same time.
Small EM transmitters often include a specially shaped and designed dielectric encapsulation which enables wave compression by a factor which comprises the frequency range only few times of the fundamental resonance frequency of the transmitter. The degree of the wave compression and therefore of the frequency lowering factor is commonly limited by the material parameter used in the dielectric resonant cavity antennas.
For example, U.S. Pat. No. 3,823,403, (1974) to Walter et al, discloses a dielectric or ferrite multiturn loop antenna which has a relatively high radiation resistance in the GHz range of frequencies. The high frequency of radiation has an advantage of a high density of the information transmission due to enhanced bandwidth but often time has a disadvantage of a limited penetration depth if there are objects around where the skin depth is relatively small compared to the dimensions of the objects. Also, the wave processes similar to the Rayleigh scattering on the fluctuations of the density of the surrounding medium of the otherwise relatively large skin depth and related diffraction phenomena may also contribute to the limited extension of the wave propagation. On the other hand, the low frequency radiation offers a method of the EM transmission which is free of the indicated drawbacks of the high-frequency radiation due to the enhanced skin-depth and therefore penetration extension and diminished diffraction processes.
U.S. Pat. No. 5,541,610 (1996) to Imanishi et al, discloses a antenna for a radio communication apparatus employing a chip inductor based antenna which includes a multilayered miniaturized chip inductance element having an approximately λ/4 wavelength which achieves a half-wave dipole antenna performance together with a ground having an approximately λ/4 wavelength. In a preferred embodiment, the inductance element is formed of a plurality of thin sheets of insulating material carrying conductor segments which are connected through via-holes in the sheets to form a spiral inductance element within the stack of sheets. Direct connection avoids impedance matching circuit insertion loss and low-cost miniaturization with reduced antenna gain deterioration from surrounding conductors is provided for an effective miniature portable radio communication apparatus.
U.S. Pat. No. 6,046,707 (2000) to Gaughan, et al. discloses a ceramic multilayer helical antenna for portable radio or microwave communication apparatus. A small and durable antenna for use with radio and microwave communications is formed as a helical conductor contained in a multilayered non-ferrite ceramic chip. The dielectric constant of the ceramic is selected to match the antenna to its operating frequency, which may be in the range of 0.5 to 10.0 Gigahertz. A process for making such antennas is also disclosed.
Low profile antenna performance enhancement is often achieved by utilizing engineered electromagnetic materials.
In one such realization, an integrated planar antenna printed on a compact dielectric slab having an effective dielectric constant is described in U.S. Pat. No. 6,509,880 by Sabet et al. Design of antenna elements with significant front-to-back radiation ratio is usually accomplished through the use of metal-backed substrates. However, printed antennas on metal-backed substrates have limited bandwidth and efficiency. This problem stems from the fact that the radiated field from the image of the antenna's electric current, which is placed in close proximity and parallel to a PEC, tends to cancel out the radiated field from the antenna current itself. In this case, matching the antenna input impedance is rather difficult, and if a matching condition can be achieved, it would be over a relatively narrow bandwidth. To circumvent this difficulty, a reactive impedance surface (RIS) with random voids between planar slot elements and the ground metal plate via a dielectric slab, as proposed in U.S. Pat. No. 6,509,880, is used for the antenna dielectric substrate. The so designed RIS has the following major features: it provides a reflection power that enhances the antenna front-to-back ratio; RIS has the ability to serve as a resonating cavity resulting in the antenna size reduction due to reduced wavelength λ˜1/sqrt(∈μ). However, due to inherited structural design of printed micro-strips, when conducting strips are placed between the dielectric slab and the air, the resonant surface waves along the slab surface interfere with the waves generated in the dielectric resonator resulting in a reduced power efficiency of the low profile antenna. The suggested random voids in the slab to minimize the effect of the surface waves leads to a non-uniform distribution of the material parameter, reduced coupling of the radiating slot to the dielectric slab and thus the integrated planar antenna printed on a compact dielectric slab with a metallic backing has limited capability in the frequency lowering and radiation efficiency.
Recently, a magnetic metamaterial (IEEE, Transactions on microwave theory and techniques, Vol. 54, No. 1, January 2006) was reported as extremely advantageous as a substrate for antennas. Said magnetic metamaterial is naturally nonmagnetic material with metallic inclusions. The effective medium metamaterial substrate employed electromagnetically small embedded circuits (ECs) to achieve permeability and permittivity greater than that of the host dielectric. Geometric control of the ECs allowed μ and ∈ to be tailored to the application. The magnetic metamaterial exhibited enhanced μ and ∈ with acceptable loss-factor levels. The permeability of the material varying strongly and predictably with frequency, the miniaturization factor may be selected by tuning the operating frequency. Relative permeability values in the μr=1-5 range are achievable for moderately low-loss applications. Representative antenna miniaturization factors on the order of 4-7 over a moderate (approximately 10%) transmission bandwidth and efficiencies in a moderate range (20%-35%) are demonstrated with the possibility of higher efficiencies indicated.
Using the wave-compressing technology in the area of antenna elements requires an approach which should be different from the existing methods of the controlled directivity, such as using reflecting conducting plane or a coaxial array of the loops.
The problems are arising mostly at low frequencies and are listed below.
First, matching the driving-point voltage to the input impedance depends on the spacing between parasitic elements and is not efficient at low frequencies due to large wavelength.
Second, low-frequency range is not accessible without the resonant cavity as the radiator size would scale with the wavelength if no compressor is used.
Third, minimizing the multiple-scattering and thus lateral diffusion processes from the interfaces, which implies no use of abruptly changing parameters in the space. The latter translates into the continuous change of the intrinsic impedance along the enhanced directivity.
Forth, directivity gain depends on the number of elements in the multi-component loop antenna, which however adds to the overall dimension. This is inconsistent with the requirement of keeping size down.
In connection with the above, there is a continuing necessity in small antennas operating at low frequencies and having enhanced performance characteristics, including efficiency of radiation and high directivity gain.